MEXICO CITY — Mexico's top medical officer voiced optimism Thursday that swine flu has slowed in the nation hardest hit by the virus, but the World Health Organization cautioned there is no evidence the worst of the global outbreak is over.
The U.S. caseload rose slightly to 130 as hundreds of schools nationwide shut their doors, and the crisis even reached the White House, which said an aide to the secretary of energy apparently got sick helping arrange a presidential trip to Mexico.
European health ministers vowed to work quickly with drugmakers to rush a vaccine into production, but American health officials suggested inoculations could not begin until fall at the earliest.
Mexican Health Secretary Jose Angel Cordova told The Associated Press new cases have leveled off and the death rate has been nearly flat for several days. He said the next few days would be critical in determining whether the virus was truly on the decline.
"The fact that we have a stabilization in the daily numbers, even a drop, makes us optimistic," he said. "Because what we'd expect is geometric or exponential growth. And that hasn't been the situation. So we think we're on the right track."
The health ministry, which earlier said 168 people were believed killed by swine flu in Mexico, on Thursday would only confirm 12 of those deaths and would not say how many were suspected.
The World Health Organization's top flu official, responding to similarly hopeful remarks from other Mexican officials, sounded a more cautious note about whether the virus had peaked.
"For things to go up and down in a country is expected. If it didn't do that would be very unusual," Dr. Keiji Fukuda said in Geneva. "Hopefully we'll see more of the data of what's going on there. But I expect even in Mexico you will see a mixed picture."
Mexico has imposed what amounts to a five-day shutdown of the country, beginning Friday, in hopes of slowing the virus. All but the most essential government services will be suspended, most businesses have been urged to close, and Mexicans have been encouraged to stay in their homes.
On Wednesday, the WHO said swine flu threatened to become a pandemic, and for the first time it raised its threat level to Phase 5, the second-highest. Fukuda said Thursday there were no immediate signs that warranted declaring a Phase 6 pandemic.
Phase 5 means a virus has spread into at least two countries and is causing large outbreaks. Phase 6 means outbreaks have been detected in two or more regions of the world and a pandemic is under way.
The only confirmed U.S. swine flu death so far is a Mexican toddler who succumbed earlier this week in Texas. New cases of swine flu were confirmed Thursday in Europe, but no deaths have been reported outside North America.
In the United States, Vice President Joe Biden stirred concern by saying in an NBC interview that he would discourage family members from flying or even taking the subway because of the swine flu threat.
The White House insisted Biden meant to say he was discouraging nonessential travel to Mexico, and Transportation Secretary Ray LaHood was compelled to state flatly: "It is safe to fly. There is no reason to cancel flights."
The U.S. government has urged Americans to wash their hands and to stay home from work and avoid traveling if they feel ill.
An aide to Energy Secretary Steven Chu apparently got sick helping arrange President Barack Obama's recent trip to Mexico. The aide did not fly on Air Force One and never posed a risk to the president, the White House said.
The U.S. is taking extraordinary precautions, including shipping millions of doses of anti-flu drugs to states. Scientists cannot predict what a new virus might do, and the outbreak could always resurge later.
Scientists are racing to prepare the key ingredient to make a vaccine against the strain, but it will take several months before human testing can begin. Production would not start until fall.
"I don't want anybody to have false expectations," Health and Human Services Assistant Secretary Craig Vanderwagen said. "The science is challenging here." He said 600 million doses in six months was "achievable" based on a fall start.
In hopes of avoiding confusion, the WHO announced it will stop using the term "swine flu," opting for the bug's scientific name, H1N1 influenza A. Obama administration officials have also pointedly referred to the virus as H1N1 in recent days.
Switzerland and the Netherlands became the latest countries to report infections. Canada, New Zealand, Britain, Germany, Spain, Israel and Austria also have confirmed cases.
In the United States, where cases have been confirmed coast to coast, nearly 300 schools were closed Thursday, including at least 200 in Texas.
The Red Cross said it was readying an army of 60 million volunteers who can be deployed around the world to help slow the virus' spread.
Already, the looming shutdown was being felt in Mexico City. Traffic cleared in the notoriously clogged avenues, and the attorney general's office said even crime was down one-third compared with last week. Mexico City's infamous smog dropped to levels normally seen only on holidays.
Mexican President Felipe Calderon said authorities would use the five-day partial shutdown in Mexico to consider whether to extend emergency measures or ease some restrictions. The dates include a weekend and two holidays, Labor Day and Cinco de Mayo, minimizing the disruption.
Cordova told the AP that the extraordinary measures undertaken in Mexico were starting to work. Most of the Mexicans hospitalized with confirmed cases of swine flu have already been released, and he expects the suspected death toll to drop as health officials do further tests.
"Without a doubt, once we study all the cases we're going to see some where there is no evidence or justification for linking them to this virus," he said.
"I think, given the evolution this is having, given the full recovery we are seeing with treatment, there is reason to be calmer, there's reason to think that this can be solved quickly and well," he said. "We simply have a new virus with what is fortunately a low mortality rate ... so I think this problem will be resolved favorably."
Swine flu is a mix of pig, bird and human genes to which people have limited natural immunity. It has symptoms nearly identical to regular flu — fever, cough and sore throat — and spreads similarly, through tiny particles in the air, when people cough or sneeze. About 36,000 people die each year of flu in the United States.